He doesn’t know what caused his lower back to stiffen up and force him out in the second inning of the Texas Rangers’ 8-0 loss to the Astros on Tuesday.

Harrison will be examined by back specialist Drew Dossett on Wednesday in Dallas and is keeping his fingers crossed it’s nothing serious.

“Hopefully, I get good news,” said Harrison, the 28-year-old left-hander. “That’s all I can pray for and go from there.”

It’s certainly a worrisome injury for Harrison and the Rangers, and Harrison acknowledged that he’s likely going to miss at least one start.

He just hopes it’s nothing more severe after undergoing two back surgeries on a herniated disk just over a year ago that cut his 2013 season short.

“All I went through to get back, I really don’t want to have to go through that again,” Harrison said. “I’ll see what [Dr. Dossett] says and what he thinks should happen and go from there. I really would like not to have that issue again, so just praying for that.”

Harrison already had one scare during spring training. He went in healthy, but slept on a soft bed early on that caused back stiffness in a different area. That delayed his progress and he didn’t join the big-league team until April 27.

Harrison had shown signs of regaining his old form, throwing 5 1/3 scoreless innings in his last start against Colorado on Thursday. But he never found a rhythm on a breezy 66-degree night at Minute Maid Park.

The tell-tale sign of his issues was the stadium’s radar gun. He was sitting in the 86-88 mph range, touching 90 mph in the first inning, and it dropped to 84 mph in the second.

“In the second inning … just didn’t feel right,” Harrison said.

Harrison intended to pitch through it, but manager Ron Washington noticed something wasn’t right with Harrison.

So Washington, pitching coach Mike Maddux and trainer Kevin Harmon went to check on Harrison.

“He said his back was stiff and I took the ball away from him,” Washington said.

Harrison said he felt fine warming up and in the first inning when he pitched around consecutive one-out walks.

But in the second, his back tightened up on the lower left side and he gave up a leadoff single and then a two-run homer to L.J. Hoes. He retired the next two batters before giving up a double to Jose Altuve, a walk to George Springer and a run-scoring single up the middle by Dexter Fowler.

Harrison had thrown 55 pitches at that point and that’s when he informed the staff of his back stiffness.

Right-hander Justin Germano relieved Harrison and didn’t fare much better. He gave up five runs over 3 1/3 innings, including four in the fifth.

The offense, meanwhile, didn’t get much going against Astros lefty Dallas Keuchel, who struck out the side on 10 pitches in the second and got the Rangers to hit into four double plays.

Keuchel went the distance for his first complete-game shutout and also gave the Astros’ pitching staff their first shutout this season.

The story of the night, though, was Harrison and the continued run of injuries to the Rangers rotation.

It started this winter when Derek Holland tripped over his dog on a staircase, requiring microfracture surgery on his left knee and costing him at least half the season.

Then Harrison had his setback in spring followed by Yu Darvish dealing with neck stiffness that forced him to begin the year on the disabled list. Opening Day starter Tanner Scheppers (right elbow inflammation) and No. 4 starter Joe Saunders (stress fracture in left ankle) are currently on the DL, and could be joined soon by Martin Perez, who is sidelined for at least one start with left elbow inflammation.

Washington said the team would likely need to add another arm for Wednesday’s game and they would get through this rash of injuries.

“We’ll fight through it,” Washington said. “There are still games on the schedule to be played and we’re going to play them.”